The present invention relates to an engine starter system for driving an engine starter to start the engine.
Internal combustion engines used as motor vehicle power sources are normally started by a starter motor which comprises a DC series motor. Electric power is supplied from a vehicle-mounted battery to the starter motor, which is energized to cause a pinion gear mounted thereon to rotate a ring gear mounted on the crankshaft and meshing with the pinion gear. Therefore, the crankshaft is rotated to start the engine.
An electric current which is supplied from the battery to the starter motor when starting the engine is very high, e.g., 100 A or more, though it is supplied in a short period of time. Therefore, the electric power consumption by the battery is quite large. The capacity of a battery to be installed on a motor vehicle is determined primarily in view of its ability to start the engine. The large electric power which is consumed to start the engine is supplemented when the battery is charged by electric power generated by an alternator mounted on the motor vehicle and driven by the engine while the motor vehicle is running.
Batteries mounted on motor vehicles are known lead batteries as secondary batteries, and they are charged and discharged through a chemical reaction between electrodes and an electrolytic solution. Such a battery can discharge a large current within a short period of time. The battery is charged with a current of 10 A or less which is supplied over a long period of time and through a gradual chemical reaction. Therefore, if a much larger current is supplied to charge the battery, the battery would be excessively heated and the electrodes might be deformed and damaged.
Motor vehicles which are mainly used by commuters run over short distances, and motor vehicles used as delivery cars are repeatedly stopped and started highly frequently. Since these motor vehicles require the engines to be started frequently and are continuously driven over short periods of time, the batteries mounted on these motor vehicles cannot be charged sufficiently enough to make up for the electric power consumed when the engines are started. Accordingly, the batteries tend to be used up, failing to start the engines.
To solve the above problems, the applicant has proposed a motor vehicle power supply device which has a large-capacity capacitor that is charged by a battery mounted on the motor vehicle and that discharges stored electric energy to actuate the engine starter to start the engine (see U.S. patent application Ser. No. 454,267 and EPC Patent Application No. 89313559.0.
The voltage of a battery does not drop when it is discharged in a short period of time, but the voltage of a capacitor drops greatly when it is discharged. When the lubricating oil of an engine is of high viscosity and the engine is subjected to large friction, at the time the engine is started in cold climate, large electric power has to be supplied to the engine starter to start the engine. At this time, the voltage across the capacitor drops, making it difficult to start the engine. This drawback may be eliminated if the capacitance of the capacitor is increased, but there is a practical limitation on the capacitance of the capacitor.